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Half of my spray heads will not pop-up.

All,
I just moved into a new home (not new construction). When I turn on the sprinkler zone for the backyard, only about half of the spray heads pop up (Rainbird 1800, 4” risers). Upon further inspection, most of the spray bodies are buried about 1.5-2 inches below the top of the dirt. I know the previous owner had the yard re-sodded and when they laid the sod they failed to raised the spray bodies. I figured, since the sprayers were so far down in the dirt, over time the dirt and debris have ruined the sprayers and I figured I would replace them.

I did an inventory of what all I had back there. On this one zone, I have eighteen(1 Rainbird-1800, all with VAN nozzles (fourteen 15VAN, three 10VAN, and one 12VAN). Based on the spray adjustments and assuming 30 psi, I calculated a flow rate of 38 gpm. I went to the street, turned on the zone, and measured 23 gpm. The measured flow is more in line with 15 psi based on Rainbird’s tech sheets. Next, I measured the pressure on my side of the dedicated irrigation water meter. The static pressure measured 70psi. I then turned on the zone and the pressure dropped to 44 psi.

I believe I have 3/4 inch laterals. From the street, the main supply goes through an electronic controlled main shutoff valve and then 150 feet to the zone valve. My pressure measurements were between the meter and main cutoff valve.

Is it possible that I have so much pressure loss between the meter and the zone valve that I don’t have enough pressure to make the heads pop up? Or is it more likely that the spray bodies are just worn out from being buried so far down below grade?

Finally, on that zone, the spray heads that do pop up are farthest from the valve. They have enough pressure to operate.

Does sound like replacement time. However, you might disassemble one and clean it first. May just have too much packed dirt and grit in it after being buried all that time. Save you money if that works.
I'd bet on bad seals though.

It also sounds like someone is way out-drawing the water supply. One thing to try is to throttle off some of the nozzles, and see if a point is reached where the rest of the heads spray well, and the entire zone of heads pops up without leaks.

What exaclty do you mean "out-drawing the water supply"? Are you talking too much flow? If flow rate is what you are referring to, how do I determine the "target" flow rate for the zone? I said I measured 23gpm; am I shooting for 20, 15, 10 gpm?

"Zone Pressure", that is what I have been curious about all along. I may just bite the bullet and install a threaded T in the lateral after the zone valve and measure the pressure. That would tell me for sure what I'm dealing with.

If I decide to measure the pressure after the zone valve, is operational pressure all I'm interested in or do I need a static pressure as well?

More info, instead of diving into installing a fitting to allow me to measure the pressure on the active side of the zone, I replaced one head that had severe blow-by. When I turned it on, the new 4" head only came up about 3".